Air for the lungs enters the body through the mouth, which leads into the trachea. At the base of the trachea, it breaks into two bronchi which further sub-divide and branch, forming a bronchial 'tree'. Cartilage in the trachea and bronchi keep the airways open and prevent them from collapsing. The trachea has a regular arrangement of c-shaped cartilage, whereas the bronchi have irregular blocks of cartilage. The smaller bronchioles can relax using their surrounding smooth muscle, this is useful during exercise to allow a greater flow of air.

Airways in the human body have mucus lining them which helps to catch pathogens and other foreign material entering the body through the air passages. In the trachea and bronchi, it is the goblet cells of the cilliated epithelium that produces this mucus. Mucus is a slimy solution containing glycoproteins. Glycoproteins are able to trap inhaled particles e.g pathogens and dust. Chemical pollutants such as sulphur dioxide, can dissolve in the mucus to form an acidic solution that irritates the airways. The nasal passages have tiny hairs lining them as well.

Macrophages, phagocytic white blood cells patrol the surfaces of the airways remove particles such as bacteria.

The lungs provide a large surface area for gaseous exchange to take place. The lungs are are surrounded by the pleural membranes which contain pleural fluid which allows friction-free movement. This ensures the lungs can be moved by the diaphragm and the ribs (the action of breathing).

Alveoli have a very thin epithelial lining and are surrounded by many blood capillaries that carry deoxygenated blood. They provide a short distance and a large surface area over which oxygen and carbon dioxide can be exchanged. They also contain elastic fibres which expand to allow air in and recoil to help force out air. Alveoli must be kept moist to allow gasses to diffuse, this is done by a fluid produced by the wall of the alveoli. The surface tension of this fluid must be low else the alveoli may not expand when air is inhaled. To prevent this, the fluid contains a surfactant (a detergent like substance), to reduce surface tension.

Changing the depth and rate of breathing enables us to adjust our intake of oxygen and exhalation of carbon dioxide to suit our level of activity. At rest, we require on average 6.0dm3 per minute, and about 0.35 dm3 enters the alveoli with each breath.

The lungs cannot be emptied of air - at least 1.0dm3 of air will always remain, and is known as the residual volume.

Tidal volume - the volume of air breathed in and out in a single breath

Breathing rate - breaths per minute

Ventilation rate - tidal volume x breathing rate

Vital capacity - the maximum volume of air that can be breathed in and then out again.

The stroke volume from the heart is defined as the volume of blood pumped out from each ventricle during each contraction, and per minute it is called the cardiac output. The pulse is the wave caused by the stretch and subsequent recoil of the aorta, and moves all along the arteries, and is identical to the heart rate. A high stroke volume and a low resting pulse is an indication of aerobic fitness, as it only requires a small increase in pulse to achieve a much larger blood supply required for heavy exercise.

When the left ventricle contracts to force oxygenated blood out of the heart, the maximum arterial pressure during this process is known as the systolic pressure - the pressure at which the blood leaves the heart. The minimum pressure is the diastolic pressure, and it reflects the resistance of the small arteries and capillaries. This can be due to hardened arteries from atherosclerosis. Typical blood pressure is 120/80 mm Hg.

Hypertension is the condition in which both systolic and diastolic blood pressures are high at rest, and the heart is working too hard. In the short term, high blood pressure occurs because of contraction of smooth muscles in the walls of small arteries and arterioles, which is a result of the hormone noradrenaline. This hormone stimulates arterioles to contract therefore increasing the resistance, forcing the heart to work harder.

However, long term hypertension imposes a strand on the cardiovascular system and is not fully explained. If not correct, it can lead to heart failure. It has been closely linked to;

ATP, as you will remember is the energy currency for all cells - muscles need it as well, but cannot store large amounts of it, and thus it is soon exhausted during exercise. Muscles will release will utilise chemical potential energy in other molecules to make ATP for exercise that lasts longer than this. These sources include glycogen breaking down to glucose in the muscles, liver stores of glycogen being converted to glucose and fatty acids in the blood from fat stores in the body.

Respiration is the process by which new ATP is produced, and there are two forms. If there is enough oxygen, aerobic respiration occurs, where glucose, fatty acids and oxygen are broken down to form carbon dioxide and water, releasing a lot of energy in the process - some is converted to ATP, the rest is lost in heat.

This respiration usually occurs in mitochondria, with the oxygen coming from two sources - oxyhaemoglobin in the blood and oxymyoglobin store in muscle. Oxyhaemoglobin readily dissociates and releases oxygen which diffuses into muscle tissue, some is used immediately by the mitochondria, some is kept by myoglobin which acts as an oxygen store since it has a higher affinity for oxygen, thus will only release it when oxygen levels in the muscle cells is very low.

Mitochondria can not function efficiently in anaerobic conditions, but glucose can still be respired to make ATP, forming lactate in the process.

During aerobic exercise, especially if somebody starts rapidly, it can take up to four minutes for their heart and lungs to reach the oxygen demand placed on them by the muscles - and an unfit person may never be able to reach the demand and have to stop. During the time it takes for the heart and lungs to catch up, the person builds up an oxygen deficit, otherwise known as an oxygen debt - and the heavy breathing after exercising is 'paying off' that oxygen debt. It does the following;

Respiration of the lactate produced (done in the liver)

Reoxygenation of haemoglobin in the blood

Reoxygenation of myoglobin in the muscles

High metabolic rate, as the whole body is operating at above resting levels.